let foo: any = 'aaa'
foo = 123
console.log(foo.lenght);


let foo1: unknown = 'aaa'
foo1 = 123
// unknown操作前必须进行类型缩小
if(typeof foo1 == 'string') {
  console.log(foo1.length); // 不报错
}
console.log(foo1.length); // 报错


export {}